The present invention relates generally to a process and apparatus for manufacturing packing containers. More specifically, the present invention relates to a method for the folding and sealing of the longitudinal edge of a material web and an arrangement for carrying out the method.
Packing containers of the non-returnable type for the packaging of e.g. milk and other liquid dairy products are manufactured from laminated packing material which includes a carrier layer of relatively stiff material, e.g. paper, which, at least on the side which is intended to be in contact with the contents, is covered with a liquid-tight, preferably thermoplastic material, e.g. polyethylene. Beside serving as a material conferring imperviousness, the thermoplastic layer is also used for making possible the heat-sealing of the laminate. For this reason, it is often advantageous if the opposite side, that is to say the outer side, of the carrier layer is also covered with a thermoplastic material. When the packing laminate is to be converted into packing containers these are formed in a known manner by folding and sealing of the laminate, so that packing containers of the desired shape are obtained. It is of the greatest importance that the seals, which unavoidably must be present on the finished packing container, should be completely impervious to liquid. This is particulary difficult to achieve if seals of the "inside-to-outside" type are used, since the lower packing laminate edge, that is to say the edge facing towards the inside of the packing container, will then come into contact with the contents, not only with the thermoplastic-covered surface, but also with the cut edge itself where the carrier layer is exposed. In the case of a carrier layer of the fibrous type, this will gradually absorb contents, which detrimentally affects the tightness and the durability of the package. To prevent this it is known to double up the internal edge zone of the material so that the inner thermoplastic layer extends around the inner edge of the packing laminate and is sealed against the inside of the outer packing laminate. In this way contact between the contents and the carrier layer is effectively prevented so that the problem of absorption of the contents into the packing material is eliminated.
The folding of the edge zone of the material web gives rise to a doubling of the thickness of the material which in the case of most types of packing containers brings about an unacceptable increase of the thickness of the seal. For this reason a thinning of the edge zones, which later are to be doubled up, is therefore frequently performed in connection with the manufacture of the packing laminate, so that in these zones the material thickness is reduced to approximately one half to one third of the original value. The actual folding of the thinned edge zone, however, takes place only later and is carried out appropriately just before the material is converted to packing containers by passing the material through a folding arrangement. It has been found to be an advantage, if the sealing of the folded edge zone to the main part of the material web is performed jointly with the folding, since it can be assured in this way that the folding remains unaltered during the conversion of the material web to packing containers and at the same time the strength and tightness of the final seal are increased.
The folding and sealing work is made more difficult by the circumstance that only a very narrow edge zone (approx. 3 mm) is to be folded over, which owing to the small thickness of the material easily leads to the folding going wrong, so that the material is damaged. Moreover, it is difficult to achieve a durable seal of the folded-down edge zone. When the sealing of the same is done by making use of the thermoplastic layer present as an adhesive it has been found, for exemple, that the heat, which has to be applied in order to soften the thermoplastic layer, easily damages the material, so that the plastic layer is thinned out and becomes leaky in connection with the subsequent folding. Not only when the heated thermoplastic layer is used as an adhesive, but, also when an external adhesive (e.g. so-called hot-melt) is used, deposits of adhesive are formed in time on the folding tools which gradually diminish their function and jeopardize operational safety.
It is an object of the present invention to provide a method for folding and sealing a longitudinal edge of the material web, which method is not subject to the abovementioned disadvantages but can be performed at high speed and with good safety and accuracy.
This and other objects have been achieved in accordance with the invention in that a method of the type described in the beginning has been given the characteristic that the material web is fed continuously with gradual folding of an edge zone at the same time as a sealing-promoting agent is supplied to the folding area.
It is a further object of the present invention to provide an arrangement for the carrying out of the method, which arrangement is of a simple and reliable design.
This object has been achieved in accordance with the invention in that an arrangement for the folding and sealing of the longitudinal edge of a material web has been given the characteristic that it includes a number of co-operating rollers, between which the edge of the material web is arranged to pass, these rollers being profiled and provided with forming components in the shape of truncated cones whose point angles vary so that the edge zone of the material web, fed between the rollers, is gradually folded about 180.degree..
Preferred embodiments of the method as well as of the arrangement in accordance with the invention have been given the characteristics evident from the description.
The method and the arrangement in accordance with the invention make it possible to provide a secure and accurate folding. The gradual, accurately controlled folding with the help of the co-operating profiled rollers of the arrangement, practically excludes any possibility of faulty folding. The activation or the supply of adhesive only after approximately half the folding process has been carried out, effectively prevents the first components of the folding arrangement, seen in the direction of movement of the web, from being dirtied with adhesive. At the same time, the fact that the adhesive is placed (or activated) in the correct position between the already partially folded parts of the web prevents any serious dirtying of the subsequent parts of the folding arrangement, so that these too remain clean and free from deposits of adhesive. The supply of heat during the later part of the folding process also means that the risk of the heat penetrating through the material and softening even the thermoplastic layer at the back of the material is much smaller, which in turn also prevents deposits on the folding arrangement, and contributes to the packing laminate presenting a completely impervious outer plastic layer, also after the folding and sealing of the longitudinal edge zone.